Modular infrared Kepler telescope

ABSTRACT

The invention is directed to an infrared Kepler telescope which includes an objective defining an optical axis and including a positive front group and a negative rear group all arranged on the optical axis. The objective further includes an interchangeable optic interposed between the positive front group and the negative rear group with the interchangeable optic being configured to operate as a magnification changer. An ocular is mounted on the optical axis rearward of the negative rear group and the rear group and the ocular are fixedly pregiven. The positive front group is a first positive front group and is exchangeable with at least a second positive front group. The interchangeable optic is a first interchangeable optic and is exchangeable with at least a second interchangeable optic. The first and second positive front groups are optically so configured that each one of the positive front groups undercorrects spherical aberration and coma forward of the negative rear group to the same extent with or without the interchangeable optic. It is also provided that each combination has an overall diffraction-limited correction except for the distortion. The interchangeable optics (W11, W21) generate a second field of view. The telescope having interchangeable optics (W11, W21) has a negative distortion and has a positive distortion without this interchangeable optics.

FIELD OF THE INVENTION

The invention relates to an infrared Kepler telescope, that is, anafocal optical system providing an intermediate image and including anobjective and an ocular. Optics of this kind are, for example, builtinto thermal imaging apparatus.

BACKGROUND OF THE INVENTION

A modular zoom reimager which includes an interchangeablecompensator/ocular unit is disclosed in the paper of A. Mann entitled"Infrared zoom lenses in the 1980s and beyond" published in OpticalEngineering, 31(5), pages 1064 to 1071 (May 1992).

U.S. Pat. No. 5,044,706 discloses a telescope (Galilei, afocal) having afocal length (field of view) which can be changed by insertable lensgroups. Diffractive optical elements and aspheric elements are providedin the additional lenses and in the base objective.

U.S. Pat. No. 5,214,532 discloses an achromatic two-lens accessory forthermal imaging apparatus to double the range. Aspheric lens elementsare provided.

U.S. Pat. No. 5,229,880 discloses a reimager which is configured forthree fields of view by omitting or inserting a first or second two-lensgroup behind the front lens of the objective. In the embodiments shown,this two-lens group has at least one diffractive optical element. Claim9 of this patent, however, presents an embodiment without a diffractiveoptical element.

U.S. Pat. No. 5,257,133 describes a reimager having a diffractiveoptical element in the ocular as set forth in claim 1 or no diffractiveoptical element as set forth in claim 10. The use of aspheric lenses inthe objective and ocular is shown and these aspheric lenses are made ofsilicon, that is, for a 3 to 5 μm wavelength. An interchangeable opticis not provided.

U.S. Pat. No. 5,363,235 discloses a catadioptric reimager havinginterchangeable lenses for switching the image field.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a telescope of the kinddescribed above which is adapted for use in thermal imaging apparatus.It is a further object of the invention to provide such a telescopewhich, for a compact configuration with as few lenses as possible,results in a system having interchangeable components for differentfields of view and which is cost effective to manufacture.

The infrared Kepler telescope of the invention includes: an objectivedefining an optical axis and including a positive front group and anegative rear group all arranged on the optical axis; the objectivefurther including an interchangeable optic interposed between thepositive front group and the negative rear group; the interchangeableoptic being configured to operate as a magnification changer; an ocularmounted on the optical axis rearward of the negative rear group; therear group and the ocular being fixedly pregiven; the positive frontgroup being a first positive front group and being exchangeable with atleast a second positive front group; the interchangeable optic being afirst interchangeable optic and being exchangeable with at least asecond interchangeable optic; and, the first and second positive frontgroups being optically so configured that each one of the positive frontgroups undercorrects spherical aberration and coma forward of thenegative rear group to the same extent with or without theinterchangeable optic.

The afocal optical system of the invention defines an intermediate imageplane and a real exit pupil. The afocal optical system includes: anobjective having a focal length and defining an optical axis; theobjective having a structural length and being configured as atelesystem including a positive front group and a negative rear group;the positive front group and the negative rear group conjointly defininga telefactor (TF) of less than 0.7 wherein the telefactor is the ratioof the structural length of the objective to the focal length thereof;an ocular defining a focal length and including at least two lensesarranged on the optical axis; the image plane and the real exit pupildefining a distance therebetween and the distance being greater than thefocal length of the ocular by a factor of at least 1.3; and, thepositive front group being exchangeable for changing the magnificationof the system.

The afocal optical system according to another embodiment of theinvention defines an intermediate image plane and a real exit pupil andthe afocal optical system includes: an objective having a focal lengthand defining an optical axis; the objective having a structural lengthand being configured as a telesystem including a positive front groupand a negative rear group; the positive front group and the negativerear group conjointly defining a telefactor (TF) of less than 0.7wherein the telefactor is the ratio of the structural length of theobjective to the focal length thereof; an ocular defining a focal lengthand including at least two lenses arranged on the optical axis; theimage plane and the real exit pupil defining a distance therebetween andthe distance being greater than the focal length of the ocular by afactor of at least 1.3; and, an interchangeable optic having at leasttwo lenses and the interchangeable optic being insertable into theoptical axis between the positive front group and the negative reargroup for changing the magnification.

The method of the invention is for making an infrared Kepler telescopehaving a field-of-view as required. The method includes the steps of:fixedly pregiving a negative objective rear group and an ocular havingovercorrected spherical aberration and coma; and, providing a positiveobjective front group configured in correspondence to the field-of-view.

The Kepler telescope according to the invention includes an objectiveand an ocular. The objective has adapted lens components which arearranged forward of the negative rear group of the objective. Apermanent base optic of the Kepler telescope includes the negative reargroup and an interface position between the permanent base optic and theadapted lens elements is provided forward of the rear group. A specificcorrection is provided at the interface position and especiallyspherical aberration and coma are undercorrected.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein:

FIG. 1 is a Kepler telescope according to a first embodiment of themodular Kepler telescope of the invention and includes the front groupdescribed in Table 1a;

FIG. 2 is a second embodiment of the modular Kepler telescope accordingto the invention and includes the front group described in Table 2;

FIG. 3 is a third embodiment of the modular Kepler telescope accordingto the invention and includes the front group described in Table 3;

FIG. 4 shows a fourth embodiment of the modular Kepler telescope of theinvention and includes the front group described in Table 4;

FIG. 5 shows a fifth embodiment of the modular Kepler telescope of theinvention and includes the front group described in Table 5; and,

FIG. 6 shows a sixth embodiment of the modular Kepler telescope of theinvention and includes the front group described in Table 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The embodiment of FIG. 1 shows the configuration of the Kepler telescopeincluding objective Ob having converging front group F12, optionalinterchangeable optic W11 and diverging rear group H as well as theocular Ok. The ocular Ok and the rear group H conjointly define thefixed base optic B. The front group F12 and the interchangeable opticW11 are exchangeable. However, the front group F12 is selected once andis permanently connected to the base optic B. The interchangeable opticW11 can, however, be inserted during operation and is pivoted in aboutrotational axis 17 utilizing a suitable mechanical device M whereinseveral different optics are mounted of which W_(x) is representative.

The base optic B comprises the rear group H having a sphericalconverging lens made of ZnS with surfaces (7, 8) and a sphericaldiverging lens made of Germanium (Ge) with surfaces (9, 10). The focallength of the rear group H is -18 mm and this rear group is positionedso in the beam path that it effects a two-fold magnification. Theintermediate image 27 between the rear group H and the ocular Ok is notcorrected.

The ocular Ok includes two aspherical converging lenses having surfaces(11, 12; 13, 14) made of Ge. The lenses are configured to definemutually adjacent menisci. The focal length of the ocular Ok isapproximately +22 mm.

The front group F12 is a converging individual lens having surfaces (1,2) and being made of Germanium. The surface 2 of this lens is adiffractive surface.

The inserted interchangeable optic W11 includes a spherical diverginglens having surfaces (3, 4) and a converging lens having surfaces (5, 6)of which surface 6 is aspherical. Both lenses are likewise made ofGermanium.

Table 1a presents the construction data for all components of thetelescope. The exit pupil 15 is permanently pregiven having a diameterof 10 mm. The image plane 16 is shown rearward of the exit pupil 15.

The aspherical and diffractive surfaces are now described according tothe following invention.

A rotationally symmetric aspherical deformation is superposed on aspherical surface having the radius R. This deformation is given as asagitta difference pD of the aspherical surface to the spherical surfaceand is a function of the distance h from the optical axis. The basicform is a parabola in all embodiments so that the following formularesults with the aspherical constants c₁ to c_(n) :

    pD(h)=(1/2R)h.sup.2 +c.sub.1 h.sup.4 + . . . +c.sub.n h.sup.2n+2.

The diffractive groove profile is, if required, superposed and is afunction of the distance h from the optical axis and is repeatedperiodically up to the maximum groove depth T.

The diffractive cut pD(h) is computed for the embodiments as follows:

    pD(h)=T·(N(b)-integer N(h))

wherein the maximum groove depth T=WL/(n-1) and number of groovesN(h)=D₁ (h²)/WL.

In the above:

WL=blaze wavelength which is here WL=9 μm

N=refractive index of the lens

D₁ =diffractive constant.

For the above, the constants are presented in the tables.

In Table 1b, the variations of front groups F11 to F31 provided for oneembodiment are presented. The front groups F11 to F31 are shown in thelens sections of FIGS. 1 to 6 and are described in corresponding Tables1a and 2 to 6. All front groups only need two lenses with an asphericalsurface or only one lens having a diffractive surface.

The data in the column headed "Gamma" in Table 1b is the magnificationof the entire telescope and "EPD" is the entry pupil diameter. f' is thefocal length of the objective Ob with the particular front groups F11 toF31.

Two interchangeable optics W11, W21 can be combined with the above andare defined by a spherical diffracting lens having surfaces (3, 4) andan aspherical converging lens having surfaces (5, 6), both made ofGermanium. W11 has an expansion factor of 1:3.4 for a structural lengthof approximately 30 mm. W21 has an expansion factor of 1:5.4 for astructural length of approximately 45 mm. W11 is shown in FIGS. 1 and 5and is included in Table 1a. W21 is shown in FIG. 2 and is described ingreater detail in Table 2.

What is important is that in all combinations of the front groups (F11to F31) and the interchangeable optics (W11, W21), the correction of thetotal system is always good and that the spherical aberration and thecoma are both always equally undercorrected at the transition locationto base optic B.

In Tables 2 to 6, the lens data of different front groups F11 and F21 toF31 are presented. The lens data of alternative interchangeable opticsW21 are presented in Table 2. The front groups and interchangeableoptics can be combined with rear group H and the ocular Ok as desired.Other embodiments and combinations are also possible.

The distortion is influenced by introducing the interchangeable opticsW11, W21 for reducing the field of view. For example, in FIG. 1, thedistortion is negative with interchangeable optic W11; however, withoutoptic W11, the distortion is weakly positive.

The intermediate image after objective Ob is deliberately not completelycorrected; instead, individual image errors are compensated by theocular Ok.

Spherical aberration and coma are preferably almost completelydiffraction-limited corrected. The rear group H here compensates theundercorrection of the front groups F11 to F31. The astigmatism isnegative, that is, the tangential focus lies farther forward of theintermediate image plane than the sagittal focus. The image fieldcurvature is slightly positive and much less than the astigmatism. Thepupil aberration is corrected by the compensation of the front groupsF11 to F31 by the rear group H.

In the ocular Ok, the two menisci having surfaces (11, 12; 13, 14)function to keep the image field curvature low. The Petzval radius ofthe ocular Ok can thereby become significantly greater (hereapproximately seven times greater) than the focal length of the ocularOk. The outer surfaces are concave. In this way, the ocular Ok cancompensate the image errors of the objective Ob and the system remainsinsensitive with respect to manufacturing tolerances. At least oneaspherical surface is required to correct the pupil aberration. Asaspherical surfaces, the following are selected: the concave surface 11facing toward the intermediate image and the surface 14 facing towardthe exit pupil 15. This correction of the pupil aberration preventsvignetting and avoids overapertures (excess diameters) of the largefront lenses.

The configuration has an overall low sensitivity with respect tomanufacturing tolerances because the correction is less critical. Theconfiguration is cost effective because Germanium is used almostexclusively. The number of lenses as well as the number of asphericalsurfaces is minimal and no aspherical surfaces are manufactured on toxicmaterial (ZnSe, chalcogenide glasses).

In addition to Germanium, ZnS or ZnSe as well as chalcogenide glassesare utilized in individual cases because of the higher dispersion. Therear group H effects magnification of at least 1.6 and has adequate airspace so that it can be displaced longitudinally for focussing on closeobjects as internal focussing as well as also for active athermalizationin the conventional range of -40° C. to +70° C. Thus, the lenses havingsurfaces (7, 8; 9, 10) can be displaced along longitudinal axis 18 via aholder 19 in the direction of double arrow 20 for focussing. The reargroup H can also comprise only one lens.

The embodiments described are provided for the spectral range from 7.5μm to 10.5 μm and are achromatized for this purpose.

The invention, however, can also be applied in the spectral range from 3μm to 5 μm and is then based on silicon as a lens material.

It is understood that the foregoing description is that of the preferredembodiments of the invention and that various changes and modificationsmay be made thereto without departing from the spirit and scope of theinvention as defined in the appended claims.

                                      TABLE 1A                                    __________________________________________________________________________    No.                                                                              Radius                                                                             Thickness                                                                          Glass                                                                            C1   C2   C3   C4   D1                                        __________________________________________________________________________    Objective Front Group F12                                                     1  96.450                                                                             8.0  Ge --   --   --   --   --                                          2 140.827 27.6 Air  0.60e-07 -0.72e-12  0.11e-14 -0.17e-18 -0.16e-04        Interchangeable Optic W11                                                     3  -122.117                                                                           3.6  Ge --   --                                                       4  174.267                                                                            23.2 Air                                                                              --   --   (Expansion Factor 1:3.4)                            5  53.727                                                                             4.3  Ge -0.10e-05                                                                          -0.16e-10                                                  6 1163.840 9.4 Air -- --                                                    Objective Rear Group H                                                        7  -501.449                                                                           4.0  ZnSe                                                               8 -210.068 0.2 Air                                                            9 110.868 3.5 Ge                                                              10 40.794 42.0 Air                                                          Ocular Ok                                                                     11 -39.094                                                                            9.5  Ge -0.13e-04                                                                           0.19e-07                                                                          -0.18e-10                                             12 -33.874 0.2 Air -- -- --                                                   13 48.179 5.5 Ge -- -- --                                                     14 60.000 33.0 Air -0.59e-05  0.12e-07 -0.90e-11                            __________________________________________________________________________

                                      TABLE 1B                                    __________________________________________________________________________    Variant                                                                           Configuration   Gamma                                                                             EPD f'     FIG.                                                                             Table                                   __________________________________________________________________________    F11 aspherical converging lens of Ge and                                                          9.0  90 mm                                                                            appr. 200 mm                                                                         4  4                                          spherical diverging lens of ZnSe                                             F12 Diffractive converging lens of Ge 9.0  90 mm appr. 200 mm 1  1a                                                F21 aspherical converging lens of                                            Ge and 11.7 117 mm appr. 255 mm 5 5        spherical diverging lens of ZnSe                                             F22 diffractive converging lens of Ge 11.7 117 mm appr. 255 mm 2 2                                                 F31 aspherical converging lens of                                            Ge 20.0 117 mm appr. 435 mm 6 6                                                 and spherical diverging lens of                                             ZnSe                                      F23 diffractive converging lens of Ge 20.0 200 mm appr. 435 mm 3            __________________________________________________________________________                                          3                                   

                                      TABLE 2                                     __________________________________________________________________________    No.                                                                              Radius                                                                             Thickness                                                                          Glass                                                                            C1   C2   C3   C4   D1                                        __________________________________________________________________________    F22                                                                             21 125.139 10.3 Ge --  -- -- -- --                                            22 183.377 52.0 Air  0.27e-07  0.24e-12  0.71e-16 -0.73e-20 -0.13e-04                                            W21                                        23 -84.054 3.6 Ge                                                             24 343.579 35.0 Air -- --                                                     25 49.380 4.3 Ge -0.81e-06 -0.99e-10                                          26 392.240 9.4 Air -- --                                                    __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    F23                                                                             No. Radius Thickness Glass C1 C2 C3 C4 D1                                   __________________________________________________________________________    31 216.154                                                                            17.8 Ge --   --   --   --   --                                          32 318.359 129.1 Air 0.54e-08 0.46e-13 -0.14e-17 0.50e-22 -0.77e-05         __________________________________________________________________________

                  TABLE 4                                                         ______________________________________                                        F11                                                                             No. Radius Thickness Glass C1 C2 C3                                         ______________________________________                                        41   98.296   9.0      Ge                                                       42 150.469 2.5 Air +0.50e-07 -0.54e-12 +0.39e-15                              43 183.265 6.0 ZnSe -- -- --                                                  44 155.888 20.2 Air -- -- --                                                ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        F21                                                                             No. Radius Thickness Glass C1 C2 C3                                         ______________________________________                                        51   124.154  11.5     Ge   --     --     --                                    52 189.838 23.7 Air 0.25e-07 0.83e-13 0.35e-16                                53 221.920 7.0 ZnSe -- -- --                                                  54 176.808 20.7 Air -- -- --                                                ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        F31                                                                             No. Radius Thickness Glass C1 C2 C3                                         ______________________________________                                        61   189.55   20.0     Ge   --     --     --                                    62 281.113 82.4 Air 0.68e-08 0.40e-13 0.47e-19                                63 509.879 9.0 ZnSe -- -- --                                                  64 257.738 21.7 Air -- -- --                                                ______________________________________                                    

What is claimed is:
 1. An infrared Kepler telescope comprising:anobjective defining an optical axis and including a positive front groupand a negative rear group all arranged on said optical axis; saidnegative rear group having lenses defining only spherical surfaces; saidobjective further including an interchangeable optic interposed betweensaid positive front group and said negative rear group; saidinterchangeable optic being configured to operate as a magnificationchanger; an ocular mounted on said optical axis rearward of saidnegative rear group; said negative rear group and said ocular beingfixedly pregiven with an intermediate image being formed therebetween onsaid optical axis; said positive front group being a first positivefront group and being exchangeable with at least a second positive frontgroup; said interchangeable optic being a first interchangeable opticand being exchangeable with at least a second interchangeable optic;and, said first and second positive front groups being optically soconfigured that each one of said positive front groups undercorrectsspherical aberration and coma forward of said negative rear group to thesame extent with or without said interchangeable optic.
 2. The infraredKepler telescope of claim 1, wherein every combination of said positivefront group and said interchangeable optic exhibits a diffractionlimited correction of chromatic and monochromatic aberrations except ofdistortion.
 3. The infrared Kepler telescope of claim 2, wherein each ofsaid interchangeable optics generate a distinct field-of-view.
 4. Theinfrared Kepler telescope of claim 3, wherein said Kepler telescopeexhibits a negative distortion with said interchangeable optics andpositive distortion without said interchangeable optics.
 5. An infraredKepler telescope comprising:an objective defining an optical axis andincluding a positive front group and a negative rear group all arrangedon said optical axis; said objective further including aninterchangeable optic interposed between said positive front group andsaid negative rear group; said interchangeable optic being configured tooperate as a magnification changer; an ocular mounted on said opticalaxis rearward of said negative rear group; said negative rear group andsaid ocular being fixedly pregiven with an intermediate image beingformed therebetween on said optical axis: said negative rear group beingmovable along said optical axis and having sufficient air space to theother groups to facilitate a focussing of said system on a close object;said positive front group being a first positive front group and beingexchangeable with at least a second positive front group; saidinterchangeable optic being a first interchangeable optic and beingexchangeable with at least a second interchangeable optic; and, saidfirst and second positive front groups being optically so configuredthat each one of said positive front groups undercorrects sphericalaberration and coma forward of said negative rear group to the sameextent with or without said interchangeable optic.
 6. The infraredKepler telescope of claim 5, wherein every combination of said positivefront group and said interchangeable optic exhibits a diffractionlimited correction of chromatic and monochromatic aberrations except ofdistortion.
 7. The infrared Kepler telescope of claim 6, wherein each ofsaid interchangeable optics generates a distinct field-of-view.
 8. Theinfrared Kepler telescope of claim 7, wherein said Kepler telescopeexhibits a negative distortion with said interchangeable optics and apositive distortion without said interchangeable optics.
 9. The infraredKepler telescope of claim 5, wherein said air space is sufficient topermit active athermalization in a temperature range of -40° C. to +70°C.
 10. The infrared Kepler telescope of claim 5, said negative reargroup including a plurality of lenses and a holder for displacing saidplurality of lenses along said optical axis between said interchangeableoptic and said ocular.
 11. The infrared Kepler telescope of claim 10,wherein said negative rear group effects a magnification of at least1.6.
 12. The infrared Kepler telescope of claim 5, wherein said negativerear group has lenses defining only spherical surfaces.